Segmented photoplethysmographic sensor with universal probe-end

ABSTRACT

A two piece probe having a universal probe end and an interconnect cable segment provides for the use of the universal probe end with a variety of different photoplethysmographic devices. The universal probe end includes a detector, a substrate for removably affixing the probe end to a patient, a window or aperture and a connector. The interconnect cable has a connector mated to the connector on the probe end for mechanical and electrical connection of the probe-end to the interconnect cable and the photoplethysmographic monitor. The connector end of the interconnect cable also houses a plurality of emitters which are directed through the window or aperture in the probe end in order to illuminate the patient. The interconnect cable also houses the resistors or other elements which identify the type of probe, monitor manufacturer or actual emitter wavelengths. Alternatively, the emitters may be housed in the main monitor and an optical fiber may be used to direct light to the patient. An electrical conductor transmits light received by the detector back to the photoplethysmographic monitor.

FIELD OF THE INVENTION

This invention related to medical monitoring probes used inphotoplethysmographic monitors and, in particular, to a probearchitecture that enables the user of a photoplethysmographic monitor touse a universal probe-end for all monitors in combination with aprobe-interconnect cable which is designed for a specific type ofmonitor.

BACKGROUND OF THE INVENTION

It is a problem in the field of medical monitoring instruments tomanufacture a photoplethysmographic probe that satisfies a number ofdiverse and sometimes contradictory requirements. It is important thatthe probe both be simple to use and conform to a variety of patients whodiffer in size and shape. The probe must be securely affixable to thepatient, such as on a patient's appendage, without requiring complexstructures or elements that can irritate the patient. In addition, inorder to reduce the risk of infection and contamination, at least aportion of the probe should be built to be disposable so that the probeis used one or more times with the patient and can then be destroyed.The disposable portion of the probe must be inexpensive so that it canbe disposable after use and yet the patient must be shielded from anypotentially dangerous electrical signals or heat produced by the probe.The probe must also reliably and accurately perform the required bloodanalyte measurements. The probe, cable and monitoring instrument are allsubjected to a hostile environment and must be manufactured to be ruggedto survive rough handling and the presence of highly reactive fluids.

Another problem with present photoplethysmographic probes is theproliferation of probe types and monitor models. The number ofmanufacturer designs probes for use with their specific monitors. Also,the types of photoplethysmographic monitors continues to increase. Oneof the primary uses of photoplethysmography has been the monitoring ofthe oxygen saturation of a patient's blood. However, there is a desireto expand the use of photoplethysmography into the monitoring ofadditional blood analytes such as carboxyhemoglobin, methemoglobin andother dyshemoglobins. This proliferation of monitor types will addadditional confusion in the health care environment due to the number ofadditional probe types which will be offered with these new monitors.

In the specific field of pulse oximetry, the light beams are typicallygenerated by a probe using light emitting diodes (LEDs) that producelight beams at red and infrared wavelengths. Various manufacturers usedifferent wavelengths of LED's in illuminating the tissue of a patient.Additionally, many manufacturers use LED's which produce light having aspectral content characterized by a center wavelength which varies fromthe nominal wavelength of the LED. Therefore, many photoplethysmographicprobes use one or more means for identifying characteristics about theprobe being used. One common identification means is a resistor whichresides in the probe and identifies the spectral characteristics of theemitters being used in the probe thereby enabling thephotoplethysmographic monitor to utilize the correct calibration datawhen generating the blood analyte level. Another use of theidentification means is identifying the type or manufacturer of a probe.Presently, however, there are no probes which can universally be usedwith all manufacturer's phtotoplethysmograhpic monitors.

In future photoplethysmographic systems it may be preferable to uselaser diodes, which produce a beam of substantially monochromatic lightat or exceeding the light power available from light emitting diodesthat are typically used in photoplethysmography. The difficulty withlaser diodes is that their cost currently prevents them from being usedin a disposable probe. Placement of the laser diode in the monitoringinstrument necessitates the use of one or more fiber optic strands inthe cable that interconnects the disposable probe with the monitoringinstrument. The cable in a hospital environment typically suffers roughhandling and the life of the fiber optic strands in the connector cablecan be fairly limited, thereby increasing the effective cost of thedisposable probe since the cable must typically be replaced on a fairlyfrequent basis.

Alternatively, the laser diodes can be placed in a segment of theinterconnect cable between the monitor and the patient end of the probe.This, however, still requires that the segment of the interconnect cablehousing the laser diodes be reusable in order to reduce costs.

SUMMARY OF THE INVENTION

The above-described problems are solved and a technical advance achievedin the field of medical monitoring instruments by the apparatus of thepresent invention which makes use of a universal probe-end that can beattached to a plurality of monitor specific interconnect cables. Thisenables the health care provider to stock only one type of disposableprobe-end. The reusable manufacturer specific interconnect cable canremain attached to the monitor with which it is being used until thereusable cable requires replacement. In the preferred embodimentdisclosed herein, the disposable probe-end has a u-shaped channel forreceipt of a u-shaped interconnect cable end portion which houses eitherthe required LED's, laser diodes or optical fiber and mirrors forilluminating the tissue of a patient. The disposable probe-end mayinclude a window in order to protect the patient from excess heat beingemitted form the light sources and to protect the reusable interconnectcable from contamination. The required identification means is housed inthe reusable interconnect cable, thus, the disposable probe-end can havetruly universal application across all photoplethysmographic monitors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top perspective view of the disposable probe-end of anembodiment of the present invention.

FIG. 2 is top perspective view of the interconnect cable segment of anembodiment of the present invention.

FIG. 3 is a bottom perspective view of the interconnect cable segment ofone embodiment of the present invention.

FIG. 3b is a bottom perspective view of the interconnect cable segmentof an alternative embodiment of the present invention.

FIG. 4 is a top perspective view of the combined probe-end andinterconnect cable segment of an embodiment of the present invention.

FIGS. 5 and 6 illustrate the two types of photoplethysmographicmonitors.

DETAILED DESCRIPTION

The apparatus of the present invention represents aphotoplethysmographic probe architecture which will reduce theproliferation of disposable probe types and result in cost-savings andease of use for the health care provider.

One type of photoplethysmographic monitor, a pulse oximeter, isfrequently used to monitor the condition of a patient in a hospitalsetting. The pulse oximeter instrument noninvasively measures the oxygensaturation of the arterial blood and produces a human readable displaythat indicates both the patient's heart rate and the oxygen saturationof the arterial blood. These readings are important to enable themedical staff to determine whether the patient's respiratory system isfunctioning properly, supplying sufficient oxygen to the blood.

A pulse oximeter instrument operates by use of a probe that illuminatesan appendage of the patient (such as a finger, earlobe, toe, neonatalappendages, or the nasal septum) that is rich in arterial blood andmeasures the differential absorption of the light by the pulsatileportion of the arterial blood flow to thereby determine oxygensaturation of the arterial blood. The pulse oximeter instrument makesuse of a plurality of light-emitting devices, each of which transmitslight at a predetermined wavelength, which wavelengths are selected suchthat at least one is highly absorbed by oxygenated hemoglobin in thearterial blood and at least one is highly absorbed by reduced hemoglobinin the arterial blood. The amount of absorption of the light beamsgenerated by these light emitting devices that are located in the probeis a measure of the relative concentration of the various hemoglobinspecies contained in the arterial blood. The absorption of the lightthat illuminates the appendage of the patient includes a constantportion that is a result of skin, bone, steady-state (venous) blood flowand light loss due to various other factors. The pulsatile component ofabsorption is due to the pulsatile arterial blood flow and is a smallfraction of the received signal and is used by the pulse oximeterinstrument to perform its measurements. It is also possible to measureadditional analytes in the arterial blood, such as additionaldyshemoglobins, with one additional wavelength of light for eachcomponent.

The measurements are computed by sampling the output of the lightdetector located in the probe to determine the incremental change inabsorption of the various wavelengths of light that are used toilluminate the appendage of the patient. These incremental changes inlight absorption are then used to compute the oxygen saturation of thearterial blood as well as the patient's pulse rate. Since the pulsatilecomponent of the signals received by the light detector represent only asmall fraction of the incident light, it is important that the incidentlight be of significant magnitude to result in transmitted signals thathave sufficient amplitude to provide accurate readings. In addition, theprobe containing the light-emitting devices and the light detector mustbe placed in intimate contact with the skin of the patient to obtain themost accurate readings.

Referring to FIGS. 1, 2, 3, 4, 5 and 6, the probe 40 of the presentinvention is designed for use with one of two basicphotoplethysmographic monitor architectures depicted in FIGS. 5 and 6.FIG. 5 depicts one of the monitor architectures in which the monitor 100communicates with probe 40 through socket 104 and mating interconnectcable plug 42. The configuration of plug 42 depends on the type andmanufacturer of monitor 100. For example, if the monitor 100 is capableof measuring a plurality of blood analyte concentrations such as O2Hb,RHb, COHb and MetHB then plug 42 and interconnect 104 will containsufficient electrical connectors 46 to interconnect probe interface 102with a plurality of emitters 21, 22, 23 and 24 located in theconnector-end 25 of interconnect cable segment 20. In a preferredembodiment, the plurality of emitters 21, 22, 23 and 24 each emit lighthaving a distinct spectral content characterized by a distinct centerwavelength denoted by λ₁ λ₂ λ₃ and λ₄. These emitters may belight-emitting-diodes (LED's) or laser diodes. It is also possible tofilter a broadband light source to produce light having four spectralpeaks of differing wavelengths. In a photoplethysmographic instrumentdesigned to generate four blood analyte levels, the preferred embodimentis to use at least four separate emitters each producing light with adistinct spectral content. If fewer blood analyte levels are desiredthen fewer emitters may be used either in the interconnect cable. Forexample, in a probe made according to the present invention for use witha standard pulse oximeter, only two emitters would be needed in theinterconnect cable.

Probe identifier 47 is located in cable 28, for example in plug 42, andprovides a means for identifying the type or family of the probe (ear,finger, toe etc.) or a means for identifying the manufacturer of theprobe or a means for identifying the actual center wavelength of each ofthe plurality of emitters in the interconnect cable. The configurationof plug 42 and probe identifier 47 will be dictated by thephotoplethysmographic monitor for which interconnect cable 28 isdesigned. Probe identifier 47 may be a resistor or set of resistors, adiode or set of diodes or some other electrical identifier. It is alsopossible that more than one identier may be necessary if, for example,information regarding both probe family as well as actual emitterwavelength must be communicated from the interconnect cable to thephotoplethysmographic monitor.

The intensity of light transmitted through the tissue under test ismeasured by one or more photodetectors 13 which are located in thedisposable probe-end 10. Photodetector 13 provides a signalcorresponding to the intensity of light received denoted I.sub.λ1,I.sub.λ2, I.sub.λ3 and I.sub.λ4. This signal is then electrically routedback to the monitor 100 through cable 28, plug 42 and socket 104 to theprobe interface 102. In the monitor the analog received intensitysignals, I.sub.λ1, I.sub.λ2, I.sub.λ3 and I.sub.λ4 are converted intodigital signals through a well-known analog to digital (A/D) converter.The intensity signals are then stored in memory 106 and manipulated indata processing circuit 107 of the monitor 100 according to dataprocessing instructions stored in memory 106 and executed by the dataprocessing circuit 107 in order to determine an estimate of the bloodanalyte levels output as a percentage concentration. Blood analytelevels (output as percentages) may then be displayed via display driver109 and graphic display 114 and/or numeric display 115.

An alternative method of implementing a photoplethysmographic monitor isdepicted in FIG. 6 and operates in conjunction with the alternativeprobe embodiment in FIG. 3b. The emitters 21, 22, 23 and 24 (which maybe LED's or laser diodes, but which are preferably laser diodes) arehoused in the monitor 200. The emitters 21, 22, 23 and 24 are driven byemitter driver 230 which is controlled by the data processing circuit107. The emitted light is then combined by an optical coupler 228 andtransmitted to the probe 40 by internal optical fiber 260 which iscoupled to an optical fiber or other such optically transmissivematerial (not shown) internal to interconnect cable segment 28. Thus,plug 42 and connector 204 now includes electrical conductors forconnecting photodiode 13 back to the probe interface 102 and an opticalconnector for connecting optical fiber 260 to an optical fiber (notshown) located in cable 28. The light transmitted through the opticalfiber 260 and transmitted through the optical fiber located in cable 28,reflects off mirror 29 of FIG. 3b in connector end 25 and passes throughwindow or opening 26 and window 16 in probe-end 10 and impinges on thetissue of the patient. Transmitted light is received by photodetector 13which send an electrical signal back to the probe interface 102 throughthe electrical conductors in cable 28 and electrical pathway 262 inmonitor 200. The probe interface 102 converts the analog signal to adigital signal and transmits the signal to memory 106 and on to dataprocessing circuit 107 for processing and display of blood analyteconcentration values using graphic display 114 and numeric display 115driven by display driver 109. The transmission of the emitted light canbe controlled in a time division multiplexing manner and an opticalcoupler 228 can be used to provide a means for combining multipleemitters onto one strand of optical fiber 260. Alternatively, theplurality of emitters may be placed or abutted against optical fiber260.

The Probe

Referring again to FIG. 4 the probe 40 consists of two releasableconnected segments: the probe-end 10, separately depicted in FIG. 1 andinterconnect cable segment 20, separately depicted in FIGS. 2, 3 and 3b.

Universal probe-end 10 is designed to be disposable and to accommodateany type of interconnect cable segment 20, whether it contains LED's,laser diode, or an optical fiber/mirror for illuminating the tissue.Thus, the construction of probe-end 10 is simple and flexible consistingof a flexible substrate 11 having three portions 12a, 12b and 12c.Substrate portions 12a and 12b are designed to wrap around an appendageof a patient. In FIG. 1 the substrate is designed to be wrapped around afinger. Substrate portions 12a and 12b have an adhesive layer applied toone or more surfaces to enable the portions to be removably attached tothe finger. Substrate portions 12a and 12b may be configured differentlyfor attachment of probe-end 10 to the feet of neonates, the smallerfingers of children, or to other appendages of adult, child or neonatalpatients.

Substrate portion 12c includes a photodetector 13 which is mounted in anoptically transmissive window 19 in the substrate 11 and which isconnected to connector 17 through flexible electrical circuits 14 a-d.Connector 17 has three lip portions 17a, 17b, and 17c which are designedto accommodate and hold the connector-end 25 of interconnect cablesegment 20. Connector portion 15 provides a larger surface area forconnector 17 to be either adhesively affixed to substrate 12 or,alternatively, to be retained between layers of substrate 12. Connector17 also houses an optically transmissive window 16 and electricalcontacts 18a, 18b, 18c, and 18d which connect flexible electricalcircuits 14a,b,c and d with interconnect cable segment 20.

Window 16 enables light to be passed from the emitters 21,22,23 and 24or optical fiber and mirror 29 to the patient while protecting thepatient from excess heat and also protecting the reusable interconnectcable segment 20.

FIG. 2 depicts a top elevational view of the patient end of interconnectcable segment 20. Cable 28 consists of a bundle of one or moreelectrical conductors and, if the emitters are located in the monitor200, an optical fiber. The electrical conductors and optical fiber arecovered by a flexible insulating material to protect the internalcomponents. Conductive grooves 28a, 28b, 28c and 28d provide forelectrical interconnection of photodetector 13 through flexibleconductors 14a, 14b, 14c and 14d and electrical contacts 18a, 18b, 18cand 18d. Ridge 27b is designed to fit under lip 17b to holdconnector-end 25 onto probe-end 10.

FIG. 3 depicts a bottom elevational view of one embodiment of thepatient end of interconnect cable segment 20. The embodiments of FIG. 3includes four emitters 21, 22, 23 and 24. Such an embodiment would beuseful in a photoplethysmographic monitor for measuring three or fourblood analytes such as oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobinand methemoglobin. Alternatively, the number of emitters could be two ina probe for use with standard pulse oximeters which typically use twoemitters having spectral contents characterized by distinct centerwavelengths, often 660 nm and 940 nm. Ridges 27a, 27b and 27c fit underlip portions 17a, 17b and 17c to hold connector-end 25 onto probe end10. The end of latch 30 is designed to click into a hole near the baseof lip portion 17c so that upon engagement of the connector-end 25 intoconnector 17 a releasable latching occurs. In order to disengage thelatching mechanism latch 30 is simply depressed.

FIG. 3b depicts essentially the identical interconnect cable segment 25with the exception that emitters 21, 22, 23 and 24 have been replaced bymirror assembly 29 which reflects light from the optical fiber enclosedin cable 28 which ends near opening 26 through window 16 and onto thetissue of the patient.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. For example, itshould be appreciated that the method and apparatus as taught by thepresent invention may be modified in an unlimited number of ways withinthe framework of the teachings of the present invention. Thesevariations are all considered to fall within the scope of the presentinvention. Therefore, it is to be expressly understood that suchmodifications and adaptations are within the spirit and scope of thepresent invention, as set forth in the following claims.

I claim:
 1. A probe for illuminating tissue of a subject to measurelight absorption of said tissue by a photoplethysmographic measurementsystem, comprising:a universal probe-end and an interconnect cablesegment having a connector-end; said universal probe-end comprising:aflexible substrate for removably affixing said probe-end to said tissue,wherein a first side of the flexible substrate is affixable to contactsaid tissue; a connector, fixedly attached to a second side of saidsubstrate, adapted to receive said connector-end of said interconnectcable, wherein said second side of the substrate is opposite to saidfirst side; a plurality of electrical contacts housed by said connector;an optically transmissive window housed by said connector; aphotodetector mounted in the flexible substrate; and a first pluralityof flexible electrical conductors extending between said photodetectorand said plurality of electrical contacts; said interconnect cablesegment comprising a plurality of electrical and/or opticallytransmissive conductors and a plug at the end opposite saidconnector-end adapted for electrical and/or optical attachment of saidplurality of electrical and/or optically transmissive conductors to oneor more specific photoplethysmographic measurement systems, wherein saidconnector-end further comprises:a housing having an aperture on onesurface, wherein said aperture is aligned with said opticallytransmissive window upon interconnection of said connector end to saiduniversal probe-end; a light source located internal to said housing andarranged so as to emit light through said aperture, wherein theoptically transmissive window of the universal probe enables the lightto pass therethrough to the tissue while protecting the connector-end ofthe interconnectable cable segment from contamination when the connectorend is interconnected to the universal probe-end; and a second pluralityof electrical contacts on said connector-end of said interconnect cableadapted to contact said first plurality of contacts, wherein said lightsource is electrically and/or optically connected to said plurality ofelectrical and/or optically transmissive conductors, and wherein saidsecond plurality of electrical contacts are electrically connected tosaid plurality of electrical conductors upon interconnection of saidconnector end to the universal probe-end.
 2. The probe of claim 1wherein said light source internal said interconnect cable segmenthousing is a plurality of light emitting diodes and, said plurality oflight emitting diodes is electrically connected to saidphotoplethysmographic measurement system.
 3. The probe of claim 1wherein said light source internal said interconnect cable segmenthousing is a plurality of laser diodes and, said plurality of laserdiodes is electrically connected to said photoplethysmographicmeasurement system.
 4. The probe of claim 1 wherein said light sourceinternal said interconnect cable segment housing is an optical fiberadapted to be illuminated by a plurality of emitters located inside saidphotoplethysmographic measurement system.
 5. The probe of claim 4,further comprising a mirror located internal said housing of saidconnector-end and adapted to reflect light from said optical fiberthrough said aperture in said housing.
 6. The apparatus of claim 1wherein said connector of said probe-end comprises:a u-shaped lipadapted to receive the apertured surface of said connector-end.
 7. Theapparatus of claim 6, wherein said connector end further comprises au-shaped ridge adapted to be received under said u-shaped lip of saidconnector of said probe-end.
 8. The apparatus of claim 6, wherein saidconnector further comprises a latch for releasable latching uponengagement of said connector end into said connector.
 9. A probe forilluminating tissue of a subject to measure light absorption of saidtissue by a photoplethysmographic measurement system, comprising:auniversal probe-end and an interconnect cable segment having aconnector-end; said universal probe-end comprising:a flexible substratefor removably affixing said probe-end to said tissue; a connector,fixedly attached to one side of said substrate, adapted to receive saidconnector-end of said interconnect cable; a plurality of electricalcontacts housed by said connector; an optically transmissive windowhoused by said connector and said substrate; a photodetector Mounted inthe flexible substrate; and a plurality of flexible electricalconductors extending between said photodetector and said plurality ofelectrical contacts; said interconnect cable segment comprising aplurality of electrical and/or optically transmissive conductors, a plugat the end opposite said connector-end adapted for electrical and/oroptical attachment of said plurality of electrical and/or opticallytransmissive conductors to one or more specific photoplethysmorgraphicmeasurement systems, and means for identifying one or morecharacteristics of the probe to said photoplethysmographic system, andwherein said connector-end further comprises:a housing having anaperture on one surface, wherein said aperture is aligned with saidoptically transmissive window upon the connection of said connector endto said universal probe-end; a light source located internal to saidhousing and arranged so as to emit light through said aperture, whereinthe optically transmissive window of the universal probe enables thelight to pass therethough to tissue of a subject while protecting theconnector-end of the interconnectable cable segment from contaminationwhen the connector end is interconnected to the universal probe-end; asecond plurality of electrical contacts on said connector-end of saidinterconnect cable adapted to contact said first plurality of contacts,wherein said light source is electrically and/or optically connected tosaid plurality of electrical and/or optically transmissive conductors,and wherein said second plurality of electrical contacts areelectrically connected to said pluarality of electrical conductors uponinterconnection of said connector end to the universal probe-end. 10.The probe of claim 9, wherein said characteristics of the probe areselected from the group consisting of probe type, photoplethysmographicsystem manufacturer and actual emitter wavelength.
 11. A probe apparatusfor illuminating tissue of a subject to measure light absorption of saidtissue by a photoplethysmographic measurement system, comprising:auniversal probe end and an interconnect cable segment having aconnector-end; said universal probe-end comprising a photodetector, aconnector adapted to receive said connector-end of said interconnectcables and an electrical conductor for connecting said photodetector tosaid connector, and wherein said probe-end has a opening aperturetherein; said interconnect cable segment including:a plurality ofemitters for emitting light through said aperture; a plurality ofelectrical conductors connected to said emitters and connectable to saidconnector; a plug at the end opposite said connector-end adapted toelectrically connect said plurality of electrical conductors to one of aplurality of photoplethysmographic measurement systems; and means foridentifying a characteristic of the probe apparatus to saidphotoplethysmographic system.
 12. The probe apparatus of claim 11wherein the characteristic of the probe apparatus is selected from thegroup consisting of probe type, photoplethysmographic systemmanufacturer and actual emitter wavelength.
 13. The probe apparatus ofclaim 12 wherein the identification means comprises one or moreelectrical elements.
 14. The probe apparatus of claim 13 wherein theidentification means comprises a plurality of resistors indicative ofthe actual wavelength of each of the plurality of emitters in saidinterconnect cable.
 15. The apparatus of claim 11, wherein theidentified characteristic is employable by the photoplethysmographicsystem to select a calibration curve for measurement determinations. 16.A probe apparatus for illuminating tissue of a subject to measure lightabsorption of said tissue by a photoplethysmographic measurement system,comprising:a universal probe-end and an interconnect cable segmenthaving a connector-end, said universal probe-end comprising aphotodetector, a connector adapted to receive said connector-end of saidinterconnect cable, and an electrical conductor for connecting saidphotodetector to said connector, wherein said probe-end has an aperturetherein; said interconnect cable segment including:a mirror located soas to direct light through said aperture, an optical fiber for directinglight from a photoplethysmogmaphic measurement system onto said mirror;an electrical conductor connected to said connector; a plug at the endopposite said connector-end for optically and electrically connectingsaid optical fiber and said electrical conductor to one of a pluralityof photoplethysmographic measurement systems; and a means foridentifying a characteristic of the probe apparatus to saidphotoplethysmographic system.
 17. The probe apparatus of claim 16,wherein the characteristic of the probe apparatus is selected from thegroup consisting of probe type, photoplethysmographic systemmanufacturer and photodetector type.
 18. The probe apparatus of claim16, wherein the identification means comprises one or more electricalelements.
 19. The probe apparatus of claim 18, wherein theidentification means comprises a plurality of resistors indicative ofthe actual wavelength of each of the plurality of emitters in saidinterconnect cable.
 20. The probe apparatus of claim 16, wherein thecharacteristic of the probe apparatus is selected from the groupconsisting of probe type, photoplethysmographic system manufacturer andactual emitter wavelength.
 21. The probe apparatus of claim 16, whereinthe identified characteristic is employable by the photoplethysmogrphicsystem to select a calibration curve for measurement determinations.